Over the last years LCD (Liquid Crystal Display) resolution and panel size of applications like smartphones, computer displays, television sets, automotive dashboard and infotainment systems have grown with the demand for backlight solutions having increased brightness and increased illuminated area. As a result, the power required for backlight is rising, despite the fact that LEDs (being a dominant source for backlight) improved their light efficiency in parallel. As LED luminance is proportional to current, uniform brightness across a panel is typically achieved by connecting multiple LEDs in series, i.e. by using LED strings. As the supply voltage for such LED strings may be higher than the supply voltage of the application, a boost converter may be used for converting the base voltage of the application into the supply voltage for the LED strings.
A possible approach to controlling brightness of an LED string is pulse width modulation (PWM) of a duty cycle controlled switch as shown in FIG. 1. FIG. 1 shows a lighting circuit 100 comprising n LED strings (n>1, e.g. n=10, 50, 100 or more). Each LED string may be used to provide backlight for a different segment or sub-region of a display panel. The base or input voltage Vin is converted into a supply voltage Vbst for supplying the LED strings. The input voltage may be in the range of 3V (e.g. in case of a smartphone), 12V or 24V (e.g. in case of an automotive or TV panel) and the LED string supply voltage may be at 30V, 60V or more. The load current through the different LED strings is controlled using the PWM control switches S31 to S3n, respectively, wherein a PWM control switch is arranged in series with the corresponding LED string.
The PWM control switches may be switched on and off periodically with a refresh frequency or cycle rate of 100 Hz, 200 Hz, 500 Hz or more. Hence, the refresh cycle of a PWM control switch may have a duration or cycle length of 1 ms, 0.5 ms, 0.2 ms or less. The brightness of an LED string may be controlled by controlling the duty cycle of the corresponding PWM control switch, i.e. by controlling the percentage of time that the PWM control switch is on during each cycle. By adapting the duty cycle for different cycles, the brightness of an LED string may be modified for different cycles.
The load current through an LED string is dependent on the supply voltage Vbst provided by the boost converter. LEDs typically exhibit manufacturing tolerances with regards to their threshold or dropout voltage. By using LED binning, such manufacturing tolerances may be taken into account to provide LED strings which exhibit similar cumulated threshold or dropout voltages. Nevertheless, different LED strings typically still exhibit different cumulated dropout voltages, which would result in different brightness for the different LED string, when arranged in parallel to the same supply voltage Vbst, because of the different levels of load currents drawn by the different LED strings. On the other hand, brightness of parallel LED strings can be matched by adapting the PWM duty cycle of the PWM control switches for the different LED strings reversely proportional to the load current drawn by each LED string.
If the PWM control switches are used for compensating the different dropout voltages across the different LED strings and/or for compensating the different load currents through the different LED strings, in order to achieve a uniform brightness for the different LED strings, the dynamic range of the resulting display panel is reduced. The duty cycle of a PWM control switch may be adapted between 0% and X % (e.g. X=50, 60, or more, up to 100) with a resolution of Q bits (e.g. Q=8 or more). The compensation of the different dropout voltages of the different LED strings may require a certain PMW range (e.g. 15-25% of the maximum PWM regulation range), thereby reducing the maximum uniform brightness of the panel by the same amount and/or thereby reducing the remaining resolution for dynamic range control of the display panel (to less than Q bits).